A magnetic loop antenna is often preferred for the Near Vertical Incidence Sky-wave (NVIS) high frequency radio communication mode. Depending on range, terrain, ionospheric conditions and other factors, units participating in a local or regional radio communication network may need to switch to and from NVIS mode, especially when mobile units are involved. Switching between communication modes typically implicates changing the operating radio frequency range and switching between a magnetic antenna such as a magnetic loop or half loop antenna, and an electrical antenna such as a whip or wire antenna. It is well known in the art to insert a tunable impedance matching element between the radio set and the antenna to optimize power transfer at different radio frequencies.
U.S. Pat. No. 3,794,941 discloses an antenna tuner comprising a control circuit to automatically tune the impedance matching element. In a similar tuner disclosed in U.S. Pat. No. 5,589,844 the impedance of the antenna is conjugately matched to the output of the radio set amplifier output stage by using a so-called pi-type impedance matching network having shunt capacitive circuit elements and one or more series inductive circuit elements.
Those and similar known antenna tuners having series inductance and parallel (shunt) capacitance are referred to as the classical tuner in the following.
The classical antenna tuner does not work efficiently with a magnetic antenna due to its low radiating resistance, and thereby high currents. For acceptable efficiency, the sum of losses in the antenna tuner and the magnetic antenna must be kept well below losses normally accepted for a classical antenna tuner with an electrical antenna.
A simple solution is to use different tuners for the magnetic and the electrical antenna, but this means increased weight and cost, and it is highly impractical in field operations.
A tuned magnetic antenna system has a narrow efficient bandwidth and the impedance matching element must be tuned to a relatively high degree of accuracy. A variable capacitance may be integrated with the magnetic antenna, for example as disclosed in U.S. Pat. No. 5,072,233, but the complexity and cost increases and accurate tuning may be difficult. Tuners with an integrated variable capacitance for tuning to a magnetic antenna are also known in the art.
Because these systems work at a nominal fifty ohms, they have either an infinitely variable capacitor or a bank of multiple capacitors in parallel to obtain a good tune. With an array of twelve parallel capacitors organized in a binary series where the capacitance doubles for each capacitor, the minimum capacitance step between capacitance settings is 1/4096 which requires capacitance precision of the order 0.025% (2^12=4096). It is currently not possible to manufacture capacitors with this precision. Indeed, it is impossible to install capacitors with this precision, especially for small values of capacitance of the order of 1 picofarad (pF) because of the tolerances in manufacture. Therefore complicated algorithms or look up tables must be used in order to obtain the linearization of the capacitor bank after manufacture. This is well known in the art.
As the frequency is increased, the variable capacitance must be reduced according to the relationshipf=½π√LC where π is the constant pi=3.14 . . . , L is the inductance and C is the capacitance.
However, there is a limit to the minimum value of C even with all the capacitors open-circuited because of stray capacitances. Consequently, many integrated systems have a maximum operating frequency around 15 MHz.
Higher operating frequencies require an additional oscillating circuit. An inductance can be connected across the capacitance. Alternatively the size of the antenna can be reduced. Although adding an inductor may increase the maximum operating frequency to 30 MHz, the efficiency decreases due to important resistive losses in the inductor due to the large current oscillating in the circuit created by this new inductor and the capacitor and which does not contribute to the radiated energy. Obviously, reducing the size of the antenna also leads to a less efficient system.